This invention relates to a wavelength tuneable laser source, the tuning of which can be continuous, which provides the extraction of the output light beam in optimised conditions, losing a minimum of energy.
In a particular embodiment, the background noise resulting from the ASE (Amplified Spontaneous Emission) is filtered spectrally and that exhibits therefore better efficiency at the emission wavelength of the laser.
A laser usually comprises a cavity in which an amplifier medium is placed. Various means are known to output the useful or source beam.
The most usual is to provide a semi-reflector at one of the ends of the cavity. The beam transmitted by this semi-reflector is the source beam.
However, such a device does not always give satisfaction, particularly when a light beam without ASE is required. It is known that to get such a free ASE light beam, a diffraction grating for filtering the ASE has to be provided in the cavity. The output light beam has then to be extracted after its diffraction by the grating and before it is another time amplified by the amplifier medium.
It is then possible to put a semi-reflecting thin plate in the cavity to get the free ASE beam by reflection one of its faces. However this involves important energy losses by reflection on the other face of the plate.
The constraints are still more important with tunable sources with an external cavity. The tuning of the output~beam wavelength generally needs the diffraction grating to be rotated. Various means have been proposed to output a free ASE beam but they command a rotation of the direction of the output beam when the wavelength is tuned.
The object of the invention is toxpropose a tunable laser source with an external cavity in which the output of the beam is optimized and which avoids the drawbacks of the previous solutions as mentioned above.
The invention then relates to a wavelength tuneable laser source with external cavity comprising an amplifier, medium, a cavity, a diffraction grating placed in the cavity comprising two reflectors one of which is able to rotate so as to tune the wavelength and source beam extraction mechanism.
According to the invention, the source beam extraction mechanism comprises a beam splitter giving a first secondary beam and a second secondary beam, parallel to one another as well as retroreflecting mechanism redirecting each secondary beams toward the beam splitter and forming a Sagnac interferometer having a reciprocal output and a non-reciprocal output, the source beam being obtained on the Sagnac interferometer non-reciprocal out put beam.
Preferably, in various embodiments of the invention each having their respective advantages:
the extraction mechanism is placed between the amplifier medium and the grating.
the amplifier medium is a wave guide and it is associated with collimation optics that collimate the beam thereby produced.
the external face of the wave guide is totally reflecting and the non reciprocal beam is the single beam transmitted by the source.
one of the cavity reflectors is a unidimensional self-aligned reflector.
the beam splitter is self-aligned.
the diffraction grating forming with the total reflector a Littman-Metcalf system.
the reflector which is mobile in rotation is also mobile in translation to enable continuous variation of the wavelength
it comprises several amplifier guides that are offset at an angle with respect to the retroreflecting-dispersing device and enabling the emission of the source over several wavelengths.